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What is a PET Scan?

Nuclear radiology is a sub-specialty of radiology in which radioisotopes (compounds containing radioactive forms of atoms) are introduced into the body for the purpose of imaging, evaluating organ function, or localizing disease or tumors. Unlike conventional or computed radiography (such as plain x-rays and CT scans) in which x-ray beams are generated within a machine and projected through the patient, in isotope studies the radiation (gamma rays) originates from within a radiopharmaceutical (material tagged with a radioisotope) in the body.

Special detector cameras are placed close against the area of interest for a period of time, and once enough gamma rays are "seen," a computer creates an image showing where the isotope localized within the organ or body. Generally, nuclear medicine scans do not provide the level of anatomic detail seen on x-ray, ultrasound, CT, or MR images. However, correlation with other imaging, clinical information, and laboratory results helps identify and confirm disease.

See bone scan, nuclear ventriculography (MUGA or RNV), pulmonary ventilation/perfusion scan, thyroid scan, lung scan, and renal scan.

Alternative Names:
Scintillation; Radionuclide organ imaging; Radioisotope; Radioactive uptake; PET scan; Nuclear radiography; Nuclear medicine scan

How the test is performed:
A radioactive isotope needs to be introduced into the body. This may be done in several ways:

  • Through a needle into a vein (usually the inside of the elbow)
  • Through a catheter that is inserted into a vein or artery and is then guided to the organ being tested
  • Ingestion (for example, to test the thyroid, the patient swallows a pill containing radioactive iodine)
  • Subcutaneous injection (under the skin)
  • Collecting a patient's own blood from a vein, adding the radioisotope compound in a laboratory, and then injecting back into the patient

After a certain period of time has passed (ranging from a few hours to a day or more for different exams), you will be placed on a table (called a gantry) under the scanner, which may rotate around the body. You must remain still to get accurate and useful images. For some tests, a counter is placed over the organ, and the amount of radioactivity or intensity of radioactivity is recorded.

A technician interprets the information as it is transmitted to the computer and can guide the camera to specific locations to improve the imaging.

How to prepare for the test:
Inform the technician or physician of ALL medications you are currently and have recently been taking, since they may interfere with the isotopes given for the exam. Also be sure to mention any recent imaging studies involving injected contrast media (dye) and oral or rectal contrast (such as from gastrointestinal studies) since they may also interefere.

You must sign a consent form before the radioactive compound is given. You may need to fast overnight before the test. Depending on the region being scanned, you may need to wear a hospital gown. Remove jewelry, dentures, and other metal that may affect the scan by blocking the gamma rays from the detectors.

How the test will feel:
If the isotope is injected, there will be a sharp prick when the needle is inserted. If a catheter is inserted, the site of insertion is usually numbed with an anesthetic. You will first feel a prick when the needle is inserted, but you will feel little more than slight pressure or tugging during the injection of the isotope. If the isotope is ingested, the flavor of the liquid may be unpleasant, but no pain is involved.

For patients who are extremely sensitive to the isotope, there may be nausea, headache, or vomiting. Discuss allergies with the health care provider or technician before the test.

Why the test is performed:
Nuclear radiography shows the size, shape, position, and some function of the target organs specific for a particular radioisotope molecule. If another test has indicated cancer or abscess, this test can help support that diagnosis and indicate the location. Repeat examinations can be used to gauge response to therapies.

Normal Values:
What abnormal results mean:

  • Abnormal size, shape, or position of an organ
  • Change in organ function
  • Stones
  • Cancer
  • Arthritis
  • Bone fractures
  • Acute infection
  • Growths
  • Abscesses
  • Embolism (obstruction of a vessel)
  • Cysts
  • Intrauterine bleeding
  • Spinal fluid leak
  • Spleen injury
  • Swelling of the testicles
  • Blood clots


Additional conditions under which the test may be performed:

  • Anaplastic carcinoma of the thyroid
  • Autonomic neuropathy
  • Gastroparesis
  • Huntington's disease
  • Unilateral hydronephrosis

What the risks are:
Generally, the energy of emitted radiation in nuclear radiography is similar to that of x-rays used in plain films and CT scans. There is potential for cell damage and mutations in egg or sperm cells.

The target organs of the isotope in the examination may receive the majority of the radiation dose, however, the amounts used are strictly controlled and regulated to use the least amount necessary for the purpose of imaging.

The radiation doses used for TREATMENT of certain disorders (for instance, iodine for thyroid disease) are many times greater and will require additional instructions to protect others during treatment.

For radiation, the greatest concern is with pregnant or nursing women. Infants and fetuses are more sensitive to the effects of radiation because they are still undergoing organ development.

Higher doses of radiation and repeated exposures to radiation increase the risk. Radioactive materials decay (release energy and transform into non-radioactive atoms) at specific rates as the body continuously removes them. The body will filter this radiation through the lungs, kidneys, or liver, depending on the compound used. Thus, all radioisotope activity eventually stops, usually within a few days.

Risks related to injections and allergic reactions to the radioisotope exist, but are rare.

See the specific type of scan for other information.

Special considerations:
The benefits of conducting an isotope study to diagnose a potentially serious condition usually outweigh concerns about radiation or other side effects.